While the 43-year-old former “Friends” star is worth a whopping $120 million, her fiance’s bank account totals a more modest $10.5 million.

“Jen is determined to protect her fortune in the event that they split,” said a close source.

“She’s working feverishly with her lawyers to hash out the pre-nup so she and Justin can marry by the end of the year. Then they plan to start adoption proceedings so they can bring home a baby in 2013.”

Screenwriter/actor Justin popped the question on Aug. 10 – his 41st birthday – and Jen is “leaving nothing to chance due to her poor track record with men,” said the source.

After divorcing Brad Pitt in 2005, Jen had broken romances with Vince Vaughn, John Mayer and Gerard Butler – but her bank account soared.

She earned a record $1 million per episode during her last two years on “Friends,” and her current asking price is $11 million per big screen movie. She’s also reaped millions from shrewd real estate investments in New York City and Los Angeles.

Meanwhile, Justin has cashed in from smaller parts in big movies and by writing screenplays for “Tropic Thunder,” “Iron Man 2” and the summer flop “Rock of Ages.”

Jen fell for Justin in 2011 on the set of the comedy “Wanderlust,” and as The ENQUIRER reported, she stood by him despite his admission to struggling with addictions to heroin and cocaine.

“Jen has found her romantic happy ending at last,” said the source. “But she’s been around the block enough times to know that she has to be realistic.

“God forbid this marriage fails, but if it does, Jen at least wants the security of knowing she won’t be wiped out financially.”